strictBindCallApply

call, apply and bind get real signatures

Without it, these three accept any arguments at all, because their declared types say `any[]`.

Since: TypeScript 3.2
strict: In strict
In your tsconfig: "strictBindCallApply": true
Compiled with: "strict": true

The same snippet both times. Only the option changed.

With it off

"strictBindCallApply": false

function greet(name: string) {
  return name
}

export const wrong = greet.call(undefined, 42)

Compiles clean

With it on

"strictBindCallApply": true

function greet(name: string) {
  return name
}

export const wrong = greet.call(undefined, 42)

Emits TS2345

Why the compiler bothers

The check was impossible to express until variadic tuple types existed, so the standard library declared these methods with `any[]` and every call through them was unchecked. That is a strange hole to leave in an otherwise strict build: the same mistake written as a direct call is caught, and written through `.call` is not. The flag swaps in the generic signatures that do the arithmetic on the parameter tuple.

Takeaway

If variadic tuples are how it is checked, a compiler too old for them cannot check it.

Where to go next

22 options